The invention relates generally to microfluidic devices, and more specifically to the pores constructed in platforms for use in microfluidic sensors, the shape and structure of such pores, methods for creating such pores, and sensors made from platforms including such pores.
In the fabrication of microfluidic devices, it is sometimes desired to create pores in the platforms used in the microfluidic devices. U.S. Pat. No. 5,846,392 to Knoll, the contents of which are herein incorporated by reference, discloses the fabrication of miniaturized circulatory measuring chambers in a carrier platform in which the chambers taper from a wide opening at one surface to a narrower opening at the opposite surface of the carrier platform. The carrier platform disclosed in the Knoll patent is made from silicon and the chamber is etched into the silicon using anisotropic or isotropic etching processes. The wide opening of the chambers are sealed with a glass plate and the narrower openings open on a channel through which an analyte is conducted to the chambers. The techniques described in the Knoll patent form chambers that have a frustopyrimidal shape. However, the methods and materials described in the Knoll patent may not be well adapted for use in the manufacture of microfluidic devices in material platforms other than silicon.
The present invention provides a microfluidic device having a sensor platform that is formed by perforating a film, such as a plastic film of an aromatic polyimide. A film prepared in this manner (platform) can then be assembled into a sensor for use in a microfluidic device using any of the film bonding techniques which are well known in the art including heat sealing, application of adhesives or clamping. Sensors constructed in accordance with the platforms of the present invention may be capable of providing improved responsiveness, sensitivity and, in particular, improved linearity over prior art sensors used in microfluidic devices. Examples of microfluidic devices of the type in which the platforms described herein would be useful are described in U.S. Pat. Nos. 5,932,799 and 6,073,482 assigned to YSI Incorporated, and the references cited therein. As noted in those patents, the pores in the platforms may be connected by fluid flow channels to create microfluidic sensors in accordance with the present invention.
In one embodiment of the invention, frustoconical pores may be perforated in the film for use in a sensor platform by laser drilling techniques. Laser drilling of films, and plastic films in particular, generally yields pores having minimal taper angles, generally less than about 15 degrees (measured with respect to vertical). Films considered operable in connection with the present invention include polyimide films and polyester films including MYLAR(trademark). Specifically, self-bonding polyimide as described in U.S. Pat. No. 5,932,799 assigned to YSI, the contents of which are herein incorporated by reference, is considered operable in connection with the present invention. Of course, other films may be used and are considered within the scope of the present invention. Additionally, platforms and sensors incorporating such platforms in accordance with the present invention may be incorporated in microfluidic devices constructed from self-bondable polyimide films in accordance with the teachings of the ""799 patent.
One factor limiting film selection for use in sensor platforms is the ability to deposit an electrical contact on the sides of the pores utilized in such platforms. For application of platinum electrodes to polyimide, it has been found particularly effective in one embodiment to first deposit a layer of diamond-like carbon amorphous film which functions as an oxygen and moisture barrier to protect the adhesion layer which is generally formed next. This barrier layer may typically be about 200 to 800 Angstroms thick. The adhesion layer is generally a layer or a metal such as chromium or titanium and typically may be about 200 to 400 Angstroms thick. The platinum electrode may typically be about 1000 to 2000 Angstroms or greater thick.
In some embodiments of the invention, sensors produced using the perforated film platforms disclosed herein utilizing a diffusion limiting film exhibit essentially linear responses for analyte concentrations of up to about 60 millimolar (mM). In other embodiments, sensors produced using the perforated films disclosed herein exhibit essentially linear responses for analyte concentrations up to about 50 mM without the need for a diffusion limiting film. It is believed that one factor contributing to the improved linearity in the sensor responses exhibited by these embodiments is the frustoconical pore geometry, as opposed to the frustopyrimidal pore geometry disclosed in the Knoll patent. It will be apparent to one of ordinary skill in the art upon review of the description that many of the embodiments disclosed herein may be combined or modified without departing from the scope of the invention.